The present invention relates to a linearisation and modulation device, and especially to a combined digital predistortion and DDS vector modulator linearisation device for a power amplifier.
In the field of radio communication, an RF signal to be transmitted via an antenna, for example, is usually modulated from two baseband signals which are orthogonal to each other. One signal is an in-phase signal (I signal), and the other signal is a quadrature-phase signal (Q signal). These baseband signals (I and Q) are converted to a bandpass signal by an IQ mixer. The principle of such an IQ-mixer is described with reference to FIG. 6. Inside the IQ mixer the I signal is modulated with an in-phase carrier and the Q signal is modulated with a quadrature-phase carrier. The modulated signals are summed together to form a bandpass signal.
The conversion from the baseband signals to the bandpass signal can be effected directly to a radio frequency (RF) signal or first to an intermediate frequency (IF) signal and then to an RF signal. In a digital IQ mixer the signal is first converted to a digital IF signal, which is then converted to analogue form by means of a digital-analogue (D/A) converter. This analogue signal is then upconverted to an RF signal, or even firstly to a second IF signal and then to the RF signal. This is because of the limitation of speed of digital devices and DA converters. However, since the processing speed of digital devices is currently increasing, it might be possible in the future to convert signals directly into an RF signal also by means of a digital device.
Before the RF signal is supplied to the antenna, it has to be amplified by a power amplifier PA.
In such an arrangement, it is essential that the power amplifier is power-efficient and at the same time shows a very low non-linear distortion in order to prevent a degradation of the RF signal to be transmitted. However, available power amplifiers suffer from non-linear characteristics. In the frequency domain, the effect of such a non-linear power amplifier occurs as a widening of the original spectrum.
In FIG. 7, spectrums of four channels are shown, wherein channel 3 exhibits the above-mentioned widening of the spectrum. Thus, an interference with the adjacent channels 2 and 4 is caused. In a mobile communication system, this leads to a degraded communication quality for the users.
Several different solutions have been proposed to cope with this situation. One solution is the so-called predistortion method. According to this method, a predistortion signal is added to the input signal of a power amplifier. The predistortion signal is chosen such that the distortion generated in the power amplifier is cancelled. The basic structure according to this solution is shown in FIG. 8, according to which a predistorter 61 adds a predistortion signal to a use signal x(t) which is to be amplified by a power amplifier 62. The non-linear distortion generated in the power amplifier 62 is eliminated as described in the following.
The distortion caused by the power amplifier can be expressed by
y(t)=Ag(t)
where A is a linear gain and g is a distortion function.
The predistorter 61 adds a predistortion to the signal:
xxe2x80x2(t)=p(x(t)
where x(t) is the original signal, p is a predistortion function and xxe2x80x2(t) is the input signal to the power amplifier 62.
Thus, the output signal of the power amplifier 62 is
y(t)=Ag(p(x(t)))
If the function p is inverse to the function of g (p=gxe2x88x921), the resultant output signal is:
y(t)=Ag(gxe2x88x921(x(t)))=Ax(t).
Hence, the distortion is cancelled.
The predistortion can be implemented in analogue form or can be effected by digital signal processing. In the latter case, a digital-analogue converter and a band-pass filter have to be provided between the predistorter 61 and the power amplifier 62.
On the other hand, as mentioned above, in a transmitting system a modulation has to be performed, in which an intermediate frequency signal (IF signal) is generated from in-phase and quadrature-phase signals (I and Q signals). For this purpose, the I signal and the Q signal are multiplied with a sine signal and a cosine signal, respectively. This modulation is usually performed in analogue form, but recently also a digital processing of this modulation has been proposed. This is called a DDS (Direct Digital Synthesis) and vector modulation. The basic structure of the DDS and vector modulation is shown in FIG. 9. The input baseband signals IIN_BB and QIN_BB are supplied to upsampling means 71 and 72, respectively.
Upsampling means (interpolation means) include a sample rate expander and a low pass filter. For example, samples are read into the upsampling means by 4 MSPS (million samples per second) and samples are output by 16 MPSPS. This is done by first adding three zeros between the 4 MPSPS samples, so that the rate is changed to 16 MPSPS. This new sequence is then low-pass filtered in order to eliminate the image caused by the original rate. A sample-rate expansion could also be effected by repeating the original sample, but this adds a sinc-response (in the frequency domain) to the signal and in this case it must be compensated for in the following filter.
The digital baseband signals IIN and QIN output from the upsampling means 71 and 72 are multiplied by multipliers 73 and 74 by an in-phase signal (cosine signal) and a quadrature-phase signal (sine signal), respectively, which are supplied for a DDS carrier generator 76. The output signals of the multipliers 73 and 74 are added by an adder 75, and a resultant IF signal IF_OUT is output.
Conventionally, this output signal IF_OUT is converted to an RF signal and then supplied to an RF power amplifier in order to supply it to an antenna. Hence, for a whole modulation and amplifier system, a complicated structure is required wherein first the modulation and then a linearisation of the power amplifier have to performed, which can be effected by the predistortion as described above, for example. Thus, a large space is required for the whole circuit.
Moreover, due to the large number of separate units, the signals are affected by noises, especially when the above mentioned linearisation using the predistortion method is performed in an analogue form.
On the other hand, if it is performed in digital form, digital-analogue and analogue-digital converters have to be included which in addition can cause degradation of the signals.
Hence, the object underlying the present invention resides in providing a device by which the above described drawbacks are removed.
This object is achieved by a linearisation and modulation device according to the present invention for a power amplifier. The linearisation and modulation device according to the invention comprises a digital vector modulator for generating a modulated digital signal on the basis of digital baseband signals and digital carrier signals; a correction value generating means for generating an amplitude correction value and a phase correction value on the basis of said digital baseband signals and on the basis of a non-linearity distortion generated in said power amplifier arranged in a subsequent stage; an amplitude correction means for generating a corrected digital signal on the basis of said modulated digital signal and said amplitude correction value; a phase correction means for generating a corrected phase control signal on the basis of said phase correction value and an output signal of a phase control signal generation means; and a carrier signal generating means for generating said carrier signals on the basis of an output of said phase correction means.
With such a device, a compact device can be constructed in which the modulation and the linearisation are performed simultaneously.
Thus, in case the modulation and linearisation device according to the invention is applied in a transmitting system for mobile communication, for example, the system can be made very compact.
Furthermore, since the digital predistortion and the modulation are performed together in one circuit, the system is less affected by disturbances.
Further advantageous developments are defined in the dependent claims.